In the automotive industry, catalytic converters typically utilize a metal foil monolith structure which supports the catalytically active materials that convert the noxious exhaust gases of an internal combustion engine to harmless gases. Conventionally, the metal foil monolith has a metal foil substrate which is extremely thin, generally on the order of about 0.05 millimeters thick. The hot exhaust gas environment of an internal combustion engine is very severe, and subjects the metal foil to continued thermal cycling which is detrimental to the bond between the metal foil and the catalytically active materials adhered to its surface. To survive in this harsh environment, the metal foil must have excellent oxidation resistance at temperatures exceeding 1000.degree. C. In addition, the metal foil must have suitable surface properties which promote adhesion of the catalytically active materials to its surface.
Currently, the standard foil material used in the automotive production of these monoliths is an Fe-20Cr-5Al alloy. This alloy is extremely oxidation resistant particularly at elevated temperatures because it forms a protective aluminum oxide scale on its surface. As is known in the art, upon exposure to an appropriate atmosphere at elevated temperatures, the relatively high level of aluminum (about five weight percent) within alloys of this type promotes the formation of a protective layer of aluminum oxide crystals, or "whiskers", on the exposed surfaces of the metal foil. Once a suitable layer of aluminum oxide whiskers is obtained, the metal foil is further coated with a catalytically-active gamma alumina washcoat material and this coating is impregnated with a noble metal catalyst. The aluminum oxide whiskers, when present in sufficient quantities to form a dense coverage over the exposed surfaces of the metal foil, provide a suitable surface to which the gamma alumina washcoat can readily adhere.
Preferably, the layer of aluminum oxide whiskers is characterized by being continuous, slow growing and adherent, covering at least about 80 percent, and more preferably at least about 90 percent of the exposed surface. Furthermore, it is preferable that the whiskers are characterized as having a length of at least about 1.5 .mu.m and possessing an aspect ratio (the ratio between the length and width of the whiskers) of at least about 2 to 1. Notably, metal foils formed from alloys having low levels of aluminum (e.g. below 5 percent weight) are generally unable to produce aluminum oxide having adequate whisker characteristics and adequate surface coverage when oxidized in an O.sub.2 - rich atmosphere. Furthermore, oxidation of Fe-Cr-Al alloys containing 3% or less of aluminum are known to cause growth of Fe- and Cr-rich oxides which are unsuitable for adhering a catalytic washcoat.
In an effort to promote the growth of aluminum oxide whiskers on a cold-rolled Fe-2OCr-5Al alloy, U.S. Pat. No. 4,318,828 to Chapman, assigned to the assignee of the present invention, proposed a process for forming a precursor layer of aluminum oxide in an oxygen-poor environment, and then further developing a dense layer of aluminum oxide whiskers in a subsequent oxygen-rich environment, such as atmospheric air at an elevated temperature.
The oxygen-poor environment taught by Chapman contained oxygen at partial pressure (P-O.sub.2) levels of between 0.75 and 1.5 torr (0.1 and 0.2 volume percent O.sub.2, or less than about 2000 parts per million (ppm)), with the balance of the environment being inert and consisting of nitrogen, hydrogen, carbon dioxide, argon or other noble gases. Chapman maintained that the precursor aluminum oxide layer could not be formed if an atmosphere containing more than about 0.2 volume percent (2000 ppm) of oxygen was present. However, in practice oxidation atmospheres having an oxygen content of 1000 ppm or more have produced little or no whisker growth. In addition, Chapman taught that the oxygen could be introduced by the disassociation of water at the furnace temperatures taught. However, Chapman noted that test results were erratic when using a pure hydrogen environment with a trace of water vapor and having a dew point of about -600.degree. C.--equivalent to a hydrogen-to-water vapor (H.sub.2 /H.sub.2 O) ratio of approximately 100,000. Such an atmosphere is extremely dry, having a water vapor content on the order of 10 ppm or less, and has an extremely low level of available oxygen for formation of the precursor layer of aluminum oxide, corresponding to a partial pressure of oxygen (P-O.sub.2) of approximately 4.9.times.10.sup.-27 atmospheres.
Though Chapman was not directed to the problem of forming aluminum oxides with alloys having an aluminum content of less than about 3 weight percent, the automotive industry is continually seeking alternatives to the use of the Fe-20Cr-5Al alloy typically used, primarily in an effort to reduce costs. The high cost of this alloy is principally due to its relatively high aluminum content within the alloy, which significantly reduces the ductility of the material, making the alloy more difficult, and thus more expensive, to process than alloys having an aluminum content of less than about 5 weight percent.
Because of the low ductility of the Fe-20Cr-5Al alloy, relatively inexpensive processing methods such as continuous slab casting, cannot be used in manufacturing the alloy because thermally induced stresses caused by rapid cooling during the continuous casting would cause the alloy slab to crack. Therefore, to avoid cracking, the alloy is almost always ingot cast. However, this is not a preferred alternative, since the tops and bottoms of the cast ingots generally contain gross inhomogeneities and therefore must be removed prior to hot working of the material, resulting in unacceptably poor yields.
Also, the poor ductility of the material due to its high aluminum content further complicates the preparation of the cast ingot surface for subsequent hot working. In order to remove casting defects, so as to obtain a high surface quality, surface grinding must be performed on the material. It is preferred that the surface grinding operation occur at room temperature, where ingot handling and inspection are easier, which results in a markedly better surface finish. Unfortunately, this Fe-20Cr-5Al material must be surface ground at high temperatures in order to avoid ingot cracking which would result from the thermally induced stresses during cooling. High temperature grinding invariably results in a surface finish of poorer quality as compared to a surface which is ground at room temperature.
In addition, the poor surface quality of the cast ingot material coupled with its low ductility cause excessive strip breakage during cold rolling of the alloy. Typically, after final rolling of this material to produce the desired foil, only about 40-60 percent of the original cast ingot is retrieved.
Therefore, what is needed is a method for forming a suitable layer of aluminum oxide whiskers on a metal foil characterized by having an aluminum content of substantially less than five weight percent, such that the material will be capable of providing the requisite adhesion for the catalytic washcoat.